1. Field of the invention
This application claims the benefit, under 35 U.S.C. §119 of French Patent Application 0605222, filed Jun. 12, 2006.
The invention relates to a switching device with selective isolation for multimedia terminals in the field of directive antenna systems required in the new wireless communications media allowing access to broadband high-data-rate services. They are situated within frequency bands currently allocated going from a few GHz for the applications of the WLAN type (2.4 GHz 802.11b, 4.9 GHz to 5.8 GHz 802.11a, 3.5 GHz Wimax) to a few tens of GHz for links of the LMDS type (28 GHz) or the satellite type (12-14 GHz or 20-30 GHz).
2. Description of the Prior Art
With the transmission techniques of the MIMO (Multiple Input Multiple Output) type, the systems use several antennas both in the transmitter and in the receiver systems in order to transmit or receive the signals. In the receiver, diversity selection allows the antenna having the highest level of received signal to be selected by switching, thus reducing the phenomena of fading. However, this design does not use all the power available to the antennas and the gain of the network remains limited.
Another transmission technique that is widely used is formed by networks of switched multi-beam antennas, which consists of a network of antennas comprising multiple fixed beams pointing in various directions. In this case, the design is relatively simple and the receiver has only to choose a correct beam for a few seconds.
With all these designs are associated a switching device that is more or less elaborate depending on the technique chosen. These devices can be formed by switches of the SPDT (Simple Port Doubles Throw) type for the simplest configuration of 2-antenna diversity or the SP4T type or the SpnT (Simple Port (n)×Throw) type for more elaborate MIMO devices where n is the order of the diversity/number of antennas used.
However, such RF/Microwave (2.4 with 5 GHz) switches require severe constraints over a wide band of frequencies:                in terms of insertion loss, in order not to degrade the noise factor too much and hence the performance in terms of sensitivity in the receiver and in order to limit the transmission power delivered by the output amplifiers, and        in terms of isolation, in order not to degrade the gain of each of the multi-beam antennas.        
Indeed, in the case of the use of a switch with low isolation between the access points, the radiation diagram resulting from the system of antennas which is weighted by the isolation of the switch loses the desired advantage in terms of reduction of interference with the other users, in other words the capacity of the antenna system to only cover one sector of the space.
But in the case of the use of a switch with high isolation between the access points, the resulting antenna diagram is the diagram of only one of the sectors or of one of the antennas of the system of antennas. This is why a high isolation between the access points is required and the switches must be very efficient.
The RF/Microwave switches currently on the market mainly using GaAs (Gallium Arsenide) and perfectly integrable into MMIC (Monolithic Microwave Integrated Circuit) technology typically use field-effect transistors (FET), the FET most commonly used is the N channel FET, called depletion FET, which has a very low drain-source resistance in the absence of gate voltage and however allows a high drain current (Idss) to flow, with the application of a negative voltage on the gate; the electric field developed under the gate induces a pinching of the channel thus notably increasing the source-drain resistance. This voltage, called Pinch Off Voltage, is around −2 to −2.5 V. They provide isolations of about 20 dB to 25 dB between channels, but this remains insufficient in the case of sectorized antennas if it is desired to maintain a high directivity gain.
Moreover, the performance of such components is nevertheless tending to becoming more critical in terms of insertion losses and isolation with the rise in frequency of the applications (2.4 to 5 GHz).
It is known from the prior art that the isolation of a switch, for example a switch using GaAs FETs, can be improved by using the resonance of the stray drain-source capacitance Cds of the FET transistor at the frequency under consideration. FIG. 1 shows an example of such a switch. This conventional design and its effectiveness are now described:
The source of the transistor T1 is connected to the input terminal E of the switch via a capacitor C1, whereas the transistor drain is connected to the output terminal S via another capacitor C2. The gate is connected via a resistor R2 to a control input C. A resistor R1 is connected between the source and the mass. An inductor L is connected between the source and the drain of the transistor.
The DC component of the input signal is filtered by the capacitors C1 and C2. The inductor L will form a resonant circuit with the residual capacitance Cds of the transistor at the frequency under consideration. The residual capacitor is of the order of 0.1 to 0.5 pF depending on the performance characteristics of the transistor T1.
The voltage Vctrl applied to the control input C allows the opening or closing of the switch according to the value of this voltage.
Such a switch provides isolations of about 20 dB between channels. This remains insufficient in the case of sectorized antennas if it is desired to maintain a high directivity gain.
In order to overcome these drawbacks, the invention provides an isolating switch formed from first and second transistors. The first transistor providing the switching function is voltage controlled and is connected, by the centre point P of an impedance bridge formed between the drain and the source of the said first transistor, to the gate of the second transistor which is itself controlled by a feedback control signal at a pre-defined frequency.
This switch has the advantage of substantially improving the isolation of the switch, in a selective manner, by guaranteeing the isolation no longer at the level of the transmission (reception) band as a whole but directly on the scale of a channel, for example, and by providing an excellent isolation of around a minimum of 30 dB between input and output channels.
Preferably, since the multimedia terminal comprises a local channel frequency oscillator, the feedback control signal is a signal coming from the said local channel frequency oscillator of the transmission terminal.
Preferably, the switches are able to be integrated using MMIC technology.